Ammonium nitrate explosive composition



United States Patent 3,103 457 AMMQNIUM NITRATE EXPLOSWE CGWGSKTKQN AND MKGCESS FOR PREPARING IT Samuel W. Grossrnann, Petersburg, Va., assignor to Allied Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Mar. 24, 1960, Ser. No. 17,268 4 Claims. (Cl. 149-46) This invention relates to an explosive composition and more particularly to a composition adapted to be used in blasting operations, and to a process for preparing it.

It is known that certain porous types of ammonium nitrate, when mixed with combustible materials such as petroleum oil, will absorb the oil, and when the oil-impregnated ammonium nitrate is packed into bore holes, or is otherwise confined, it can be detonated with the aid of conventional primer charges.

Such porous, absorptive ammonium nitrates are usually prepared by spraying or prilling molten ammonium nitrate solutions containing from 4% to, 6% water, into a tower to produce pellets or prills, which are thereafter dried to eliminate water. Upon removal of water from the pellets, small voids are created throughout the pellets where the water has been, thus rendering the individual pellets porous and capable of absorbing sufficient oil, usually from about 3% to 6%, to produce a readily explosive mixture. The process used for preparing these porous ammonium nitrate pellets is expensive, in that it involves costly drying steps carried out after formation of the pellets. These conventional porous ammonium nitrate pellets, moreover, are quite fragile and tend to become crushed into fines and pack and cake badly in storage and shipment. The conventional porous ammonium nitrate materials, moreover, are extremely bulky, having low bulk densities, usually in the range between about 0.70 gram per milliliter and about 0.90 gram per milliliter, thus sharply limiting the quantity of explosive that can be placed in a bore hole of given dimensions.

More recently, economically advantageous processes have.been developed for producing dense, non-caking ammonium nitrate pellets having high bulk densities, for example, between about 0.93 gram per milliliter and about 1.0 gram per milliliter, such processes usually involving spraying or prilling substantially anhydrous (e.g. 99.7- 99.9%) molten ammonium nitrate into ,a cooling tower to produce pellets which require no further drying. These high bulk density ammonium nitrates, however, are considerably less porous than the low bulk density product and are capable of absorbing only small quantities of oily combustible material, usually a maximum of about 2%, so that such high bulk density ammonium nitrates, while advantageous from the point of view of economy of manufacture and use in the agricultural arts, are of limited value for explosive use. While mixtures have been made of this high bulk density ammonium nitrate with quantities of petroleum oil greater than 2% which have proved of some value as explosive mixtures, the excess, unabsorbed oil tends to run 0E and separate from the mixture if it is allowed to stand, hence such mixtures must usually be prepared at the blasting site and are not as eliicient as they would be if ammonium nitrate and oil were held in intimate contact with each other.

An object of the present invention is to provide a proc ess for preparing a dense, porous ammonium nitrate of high bulk density and of high absorptivity for hydrocar bon oils of the character of the so-called fuel oils.

A further object of the invention is to provide an explosive ammonium nitrate composition in particulate form of high degree of resistance to crushing.

A still further object of the invention is to provide a particulate, explosive ammonium nitrate composition havuse? I 2 ing high bulk density combined with high explosive qualities.

A still further object of the invention is to provide a process for preparing oil-impregnated ammonium nitrate explosive compositions of high bulk density, adequate oil content and high resistance to crushing. i

The above and other objects are accomplished according to my invention wherein -a non-porous, high bulk density, pelleted ammonium nitrate is rendered porous and oil absorptive while retaining its high bulk density and crush resistance, by heating the high bulk density ammonium nitrate existing as crystalline form IV of ammonium nitrate at a temperature between 32? C. and 84 C. for a period sufiicient to transform substantially all the ammonium nitrate to crystalline form III, and thereafter cooling the ammonium nitrate to below 32 C. (but above 18 C., the next transition temperature) and impregnating the thus treated ammonium nitrate with between about 3% and about 5% of a suitable liquid hydrocarbon, to produce an essentially dry, free flowing particulate ammonium nitrate explosive composition consisting essentially of between about 3% and about 5% liquid hydrocarbon absorbed by and dispersed within the pores oi said ammonium nitrate, said ammonium nitrate having a bulk density between about 0.93 gram per milliliter and about 1.00 gram per milliliter, having particles in the size range between 8 and 48 Tyler screen mesh of which between about and about 95% are in the range 8 to 35 Tyler screen mesh and have particle densities between about 1.5 grams per milliliter and about 1.7 grams per milliliter and such resistance to crushing as to be capable of withstanding a load of at least about 1000 grams applied to the particles between. anvils of. a testing device.

The dense, porous ammonium nitrate material used in my explosive compositions may be prepared from high bulk density, non-porous, pelleted ammonium nitrate originally produced by spraying or prilling substantially anhydrous molten ammonium nitrate (containing from about 0.3 to about 0.1% of Water, the balance ammonium nitrate) into a cooling tower and cooling the resulting pellets directly to a temperature below 32 C. (but above -l8 C., the temperature at which the next transition takes place). The cooled pellets are essentially spherical and have a hard, glazed surface. They are relatively nonporous, containing insuflicient pores to absorb diesel fuel oil to the extent of more than about 2%, and have particle densities of at least about 1.5 grams per milliliter, usually between about 1.5 grams per milliliter and about 1.7 grams per milliliter. The bulk density of the pelleted mass is at least about 0.93 gram per milliliter, usually between about 0.96 gram per milliliter and about 0.98 gram per milliliter. Due to the method of cooling, the ammonium nitrate thus produced exists in form IV of ammonium nitrate which is stable between 18 C. and +32 C.

The above-described nonporous high bulk density ammonium nitrate is rendered porous and oil absorptive while still retaining high bulk density and crush resistant characteristics according to the process of my invention. This conversion is accomplished by heating the high bulk density ammonium nitrate at a temperature between 32 C, and 84 C. for a period sufficient to transform substantially all the ammonium nitrate into form III of arm monium nitrate, which exists between these temperatures. The heating may be carried out in a conventional gas or oil-fired kiln or dryer until the temperature of the mass is between about 40 C. and about 70 C., preferably about 60 C; Usually between about 30 minutes and about minutes is sufiicient, and about 60 minutes at the preferred temperatures is usually adequate to convert 3 all the ammonium nitrate to form III, longer times being required at the lower temperatures. During conversion to form III the ammonium nitrate pellets expand slightly due to the larger specific volume of form III forming fissures in the pellets. The ammonium nitrate is then cooled below 32 C. and reverts to form IV. Surprisingly, this reversion occurs without collapsing the expanded pellets, leaving a porous, oil-absorptive, but still dense, pelleted mass, having a capacity for absorbing up to about of a liquid hydrocarbon of the character of No. 1, 2 or 4 fuel oil.

It is known that the transition of ammonium nitrate from form IV to form III causes an expansion in the volume of the ammonium nitrate, and that such critical change tends to weaken and usually completely disintegrates ammonium nitrate particles subjected to such treatment. I have made the surprising discovery, that when the particular high bulk density ammonium nitrate, as defined, is subjected to the successive heating and cooling treatments as outlined, the major proportion of the resulting particles, while being rendered porous, are not disintegrated as would be expected but retain both their high bulk density and also their superior resistance to crushing.

The resulting ponous, high bulk density ammonium nitrate has physical characteristics including bulk density substantially the same as the original untreated dense ammonium nitrate. Treatment results in only a slight increase fines, usually from about 1% in the original dense material to not more than about 4% in treated material, only about sutlicient to compensate for the slight decrease in bulk density of the individual pellets. Scneen size of the particles may range from about 8 to about 48 Tyler screen mesh with between about 80% and about 95% of the particles passing 8 mesh retained on 35 mesh (openings 2.36 mm. and 0.417 mm), respectively.

The explosive compositions of my invention are prepared by mixing with the treated high bulk density ammonium nitrate, between about 3% and about 5% of a normally liquid petroleum hydrocarbon. The hydrocarbon employed may be any suitable liquid hydrocarbon material such as those derived from petroleum, coal carbonization and distillation of shale, preferably petroleum distillates such as naphthas, kerosenes, gas oils and fuel oils. In practice I have liound that distillates boiling predominantly above about 400 F. and having a Saybolt viscosity at 100 F. between about 25 seconds and about 125 seconds are satisfactory. Examples of suitable hydrocarbons are the numbers 1, 2 and 4 fuel oils, specifications for which may be found in ASTM 396-48T and abbreviated in Chemical Engineers Handbook, J. H. Perry, McGraw-Hill, third ed. (1950), page 1569. Diesel fuel oils are also satisfactory. These oils are generally similar in physical characteristics to the regular fuel oils, but have usually been purified chemically, see Perry ibid., page 1654. The hydrocarbon can be mixed with the high bulk density ammonium nitrate in any manner which pro vides for the absorption of the oil into the pores of the ammonium nitrate to leave an apparently dry, free Howing, pelleted ammonium nitrate product, for example, by directing a fine spray of the oil onto a mass of the treated ammonium nitrate under agitation as in a rotary dryer which may be the vessel in which the heating-cooling treatment has been carried out. The theoretical optimum of combustible material which should be mixed with the ammonium nitrate for maximum explosive benefit is, of course, the stoichiometric amount required to consume the oxygen liberated by the ammonium nitrate. This depends on the carbon-hydrogen ratio in the hydrocarbon, and, inthe case of the fuel oils, amounts to between about 5% and about 6% by weight. In practice, however, stoichiometric mixtures, while providing maximum work, tend to be less sensitive to detonation than mixtures containing slightly less than the stoichiometric proportions of oil. Larger proportions of hydrocarbon decrease the sensitivity still further. In general good detonation and satisfactory work are obtained with mixtures containing ammonium nitrate mixed with between about 3% and about 5% by weight of the fuel oil. The resulting essentially dry, free flowing product has a particle size in the range between about 8 and about 48 Tyler screen mesh, with the bulk of the pellets, eg between about and about in the range 8-35 Tyler screen mesh. The oil-impregnated ammonium nitrate product of my invention has a bulk density between about 0.93 gram per milliliter and about 1.00 gram per milliliter, the oil increasing slightly the bulk density of the composite product over that of the unimpregnated material. My product is also highly resistant to crushing, andhence resistant to disintegration in shipment and storage which would tend to result in lumping and caking. Thus the individual particles of my product will withstand loads of up to about 1000 grams or more dead weight, applied between anvils, before crushing, whereas the prior art porous low bulk density ammonium nitrates similarly oil-impregnated will withstand only about grams. i

The composition of my invention is useful as a blasting explosive and may be placed in the bore hole in any conventional manner as by pouring directly into the hole or by placing the composition in plastic bags and inserting the filled bags into the holes. My composition has a blast sensitivity such that it is readily detonated by the usual primers or by the use of standard det-onating cord only, for example, a 50 grain cord, set oh? by a conventional blasting cap.

Use of my composition provides the advantages of a preformed, stable explosive product combining ammonium nitrate and hydrocarbon oil in such intimate association that substantially maximum work is produced by the explosive on detonation. The oil is stably absorbed Within the ammonium nitrate pores, and will not leach or run out or drain off as is the case with mixtures of non-porous ammonium nitrate and oil. The composition has superior crush resistance such that it is stable for storage and shipment. Its high bulk density permits filling a bore hole of given dimensions with up to 35% or more of explosive composition in excess of the amount which can be inserted when using conventional low bulk density ammonium nitrate compositions. Use of my composition in plastic bags such as polyethylene bags is convenient and is especially advantageous in loading bore holes which contain water, as water exerts relatively slight lifting force on the high density compositions, of almost the same density as water. On the other hand, water in the hole exerts a powerful lifting force on the low density compositions, making it diflicult or impossible to properly load the hole.

The term bulk density as used herein means weight per unit volume, and may be determined by the following test No. 1 which involves introducing the ammonium nitrate material into a glass cylinder having approximate inside dimensions of 35.5 centimeters length and 3.8 centimeters diameter, and an approximate volume of 250 milliliters in the graduated section. amounting to about 200 milliliters is introduced into the cylinder and after addition, the cylinder is tapped with a rubber stopper 3.8 centimeters diameter for three minutes. The bulk density in grams per milliliter is the weight of the ammonium nitrate in grams divided by the final volume in the cylinder in milliliters.

The crush resistance of the oil-impregnated ammonium nitrate pellets [of my invention may be determined by the following test No. 2 which involves selecting individual pellets of representative screen size from the sample and determining the load required to crush or break the pellets between two anvils. As apparatus for carrying out the test, a beam type compression testing device may be used, having a beam marked for direct reading of the load in grams dead weight, with provisionfor loading the beam Ammonium nitrate I at a uniform rate, and provided with steel anvils between which the pellet will be crushed. The test is carried out by placing a single pellet to be crushed between the anvils of the testing device, applying a load, and increasing the load until the pellet crushes, then recording the load in grams dead weight. At least ten pellets are tested and the average load taken.

Oil absorption may be determined by the following test 3, which involves placing fifty-gram portions of ammonium nitrate in 250 milliliter beakers and adding to each beaker a volume of No. 2 fuel oil calculated to give a desired percentage (by weight) of oil, based on the weight of ammonium nitrate. The several samples together form a series of suitable percentages, e.g. 2%, 3%, 4%, 5%, 6%. The oil added to each sample is distributed over the upper surface of the ammonium nitrate. Each mixture of ammonium nitrate and oil is stirred by hand with a glass stirring rod for 60 seconds and the appearance and flowability are noted. The greatest percentage at which the oil-impregnated pellets remain freeflowing is recorded as the oil absorption.

The following specific examples further illustrate my invention. Parts are by weight except as otherwise noted.

EXAMPLE 1 Two hundred parts of a high bulk density ammonium nitrate having a bulk density of about 0.968 gram per milliliter and a particle density of 1.56 grams per milliliter prepared by prilling a substantially anhydrous molten ammonium nitrate into a tower and immediately cooling to below 32 C., and existing as dense, hard prills in crystalline form IV, were placed in a closed vessel and heated to 40 C., and held at that temperature for four hours. The material was then cooled to 25 C. Fines in the resulting material amounted to about 4%.

The resulting material was tested for oil absorption using a No. 2 diesel fiuel oil, according to test No. 3 described above, and was found to have a maximum capacity of 4.5% oil as compared to 2% for the untreated ammonium nitrate.

EXAMPLE 2 Two 2,000 part samples of a prilled ammonium nitrate of the same character as used in Example 1 were heated with hot air in a rotary dryer until the temperature of the mass was 6 C. One of the samples was then cooled to below 32 C., i.e. to 25 C., in about an hour by passing a stream of cool air entering at about 23 C., over the material in the dryer. The other sample was cooled by removing it from the dryer, placing it in a Dewar flask at room temperature (2025 C.) and allowing it to stand until the mass had cooled to below 32 C., i.e. to 25 C. This cooling required 8 hours. Both samples were then subjected to oil absorption tests using a diesel oil (No. 2 fuel oil) and both had oil absorption of 4.5% diesel oil as compared to 2% oil absorbed by an untreated sample. This test illustrates that rate of cooling is not critical.

EXAMPLE 3 A 150 pound batch of ammonium nitrate pellets ranging in size from about 8 to 35 Tyler screen mesh with the bulk of the material about 12-14, having a bulk density of 0.96 8 gram per milliliter and particle density of 1.56 grams per milliliter was heated in a gas fired 2 by 4 foot rotary dryer until the temperature of the mass was 60 C., and maintained at this temperature for 1 hour. The ammonium nitrate was then cooled to 28 C. in 2 hours.

The resulting ammonium nitrate was tested for oil absorption by the test 3 method described above, and was found to absorb of diesel oil (No. 2 fuel oil) as compared to 2% absorption for the untreated ammonium nitrate.

Separate 425 gram samples of the treated ammonium nitrate were mixed with 3,. 4, 4.5 and 5% respectively,

of diesel oil by spraying the oil on the ammonium nitrate V pellets in a rotary dryer. The oil impregnated samples emerging from the dryer were free flowing, and apparently essentially dry, indicating the oil had been absorbed. The several samples were detonated with 32 inches of 50 grain Primacord detonating cord, set off by a numher 6 blasting cap. Results of the tests are shown in It is apparent that heat treated ammonium nitrate containing 4.5% oil produced the largest crater.

EXAMPLE 4 A representative sample of the heat-treated, highbulk density oil-impregnated ammonium nitrate prepared as described in Example 3 and containing 4.5% diesel fuel oil, having a bulk density of 0.98 gram per milliliter, and about 87% by weight of which was in the size range 8-35 mesh (Tyler), was compared for crush resistance with fuel oil-impregnated low bulk density, porous ammonium nitrate, by means of test No. 2 outlined above. The low bulk density material used for comparison was a typical commercial product prepared by prilling a molten solution of ammonium nitrate containing from 4 to 6% water and drying the resulting pellets. This low bulk density product had a bulk density of 0.73 gram per milliliter before oil treatment and a bulk density of 0.77 gram per milliliter after treatment with 5% diesel oil. About 90% by weight of the low bulk density pellets were in the size range 8-35 mesh (Tyler).

Table '11 below shows comparative tests on crushing resistance using oil-containing 8-35 mesh pellets of low bulk density and heat treated high bulk density material, respectively.

Table II Bulk Particle Heat Av. load, Number density, density, treatment grms. pellets g./m1. g./m1. treated a 0. 77 1. 23 N0 155 30 b 0.98 1. 56 Yes 1, 050 30 Fuel oil-impregnated low bulk density commercial ammonium nitrate pellets.

Fuel oil-impregnated high bulk density ammonium nitrate pellets prepared according to this example.

While the above describes the preferred embodiments bf my invention, it will be understood that departures may be made therefrom within the scope of the specification and claims.

I claim:

1. A11 explosive composition in particulate form consisting essentially of between about and about 97% of porous ammonium niuate particles and prepared by heating a particulate, crystalline form IV non-porous ammonium nitrate having a bulk density between about 0.93 gm./ml. and about 1.00 gut/ml, a particle density between about 1.5 gm./m1. about 1.7 -gm./ml., and a hydrocarbon oil absorption capacity of no more than about 2%, at a temperature above 32 C. and below 84 C. for a period sufiicient to transform substantially all the ammonium nitrate to crystalline iorm III of ammonium nitrate and thereafter cooling the ammonium nitrate to a temperature below 32 C. and maintaining said temperature for a time sufiicient to reconvert the ammonium nitrate to crystalline form IV and between about and about 3% of a liquid hydrocarbon oil, said hydrocarbon oil being absorbed on and dispersed Within the pores of said porous ammonium nitrate particles, said composition having a bulk density between about 0.93 gram per milliliter and about 1.00 gram per milliliter, said particles of said composition having sizes in the range between 8 and 48 Tyler screen mesh, of which between about 80% and about 95% are of such size as to pass an 8 mesh Tyler screen and be retained on a 35 mesh Tyler screen, and have panticle densities between about 1.5' grams per milliliter and about 1.7 grams per milliliter and high resistance to crushing.

2. The composition according to claim 1 wherein the bulk density of the composition is about 0.98 gram per milliliter.

6. The composition according to claim 1 wherein the hydrocarbon oil is a petroleum distillate having a Saybolt viscosity at 100 F. between about 25 and about 125 seconds and boiling predominantly above about 400 F.

4. The process for preparing a high bulk density explosive composition which comprises heating particulate,

form IV ammonium nitrate having a bulk density between about 0.93 gramper milliliter and about 0.98 gram per milliliter, and a particle density between about 1.5 grams per milliliter and about 1.7 grams per milliliter, at a temperature between about C. and about C. for a period suflicient to convert substantially all the ammonium nitrate to form III of ammonium nitrate, thereafter cooling the ammonium nitrate to a temperature below 32 C. maintaining said temperature for a time sufiicient to reconvert the ammonium nitrate to its crystalline form IV, and mixing said cooled ammonium nitrate with between about 3% and about 5% of its Weight of a liquid hydrocarbon.

References Cited in the file of this patent UNITED STATES PATENTS 2,398,071 Banab Apr. 9, 1946 2,590,054 Taylor Mar. 18, 1952 3,005,373 Ransom Oct. 24, 1961 3,009,801 Blackwell Nov. 21, 1961 OTHER REFERENCES Stites et a1.: University of Missouri School of Mines and Metallurgy, Bulletin No. 98, Nov. 19', and 20', 1959', pp. 47 and 10. 

1. AN EXPLOSIVE COMPOSITION IN PARTICULATE FORM CONSISTINGS ESSENTIALLY OF BETWEEN ABOUT 95% AND ABOUT 97% OF POROUS AMMONIUM NITRATE PARTICLES AND PRREPARED BY HEATING A PARTICULATE, CRYSTALLINE FORM IV NON-POROUS AMMONIUM NITRATE HAVING A BULK DENSITY BETWEEN ABOUT 0.93 GM./ML. AND ABOUT 1.00 GM./ML., A PARTICLES DENSITY BETWEEN ABOUT 1.5 GM./ML. ABOUT 1.7 GM./ML., AND A HYDROCARBON OIL ABSORPTION CAPACITY OF NO MORE THAN ABOUT 2%, AT A TEMPERATURE ABOVE 32*C. AND BELOW 84*C. FOR A PERIOD SUFFICIENT TO TRANSFORM SUBSTANTIALLY ALL THE AMMONIUM NITRATE TO CRYSTALLINE FORM III OF AMMONIUM NITRATE AND THEREAFTER COOLING THE AMMONIUM NITRATE TO A TEMPERATURE BELOW 32*C. AND MAINTAINING SAID TEMPERATURE FOR A TIME SUFFICIENT TO RECONVERT THE AMMONIUM NITRATE TO CRYSTALLINE FORM IV AND BETWEEN ABOUT 5% AND ABOUT 3% OF A LIQUID HYDROCARBON OIL, SAID HYDROCARBON OIL BEING ABSORBED ON AND DISPERSED WITHIN THE PORES OF SAID POROUS AMMONIUM NITRATE PARTICLES, SAID COMPOSITION HAVING A BULK DENSITY BETWEEN ABOUT 0.93 GRAM PER MILLILITER AND ABOUT 1.00 GRAM PER MILLILITER, SAID PARTICLES OF SAID COMPOSITION HAVING SIZE IN THE RANGE BETWEEN 8 TO 48 TYLER SCREEN MESH, OF WHICH BETWEEN ABOUT 80% AND ABOUT 95% ARE OF SUCH SIZE AS TO PASS AN 8 MESH TYLER SCREEN AND BE RETAINED ON A 35 MESH TYLER SCREEN, AND HAVE PARTICLESS DENSITIES BETWEEN ABOUT 1.5 GRAMS PER MILLILITER AND ABOUT 1.7 GRAMS PER MILLILITER AND HIGH RESISTANCE TO CRUSHING. 